1. Technical Field
The present invention relates to manufacturing of quartz glass, such as that used as an optical fiber base material, obtained by depositing silicon dioxide (SiO2) generated by a flame hydrolysis reaction. In particular, the present invention relates to a quartz glass manufacturing method and a quartz glass manufacturing apparatus using, as backup, hydrogen obtained by vaporizing liquid hydrogen.
2. Related Art
Currently used hydrogen supply equipment stores liquid hydrogen in a low-temperature storage chamber, heats and vaporizes the liquid hydrogen, and supplies the resulting gas to a device connected at a later stage. For example, when manufacturing quartz glass with a high degree of purity, techniques such as VAD or OVD may be used. Both of these techniques involve (i) generating silicon dioxide (SiO2) through a hydrolysis reaction achieved by supplying a silicide such as silicon tetrachloride (SiCl4) to an oxyhydrogen flame fueled by supplying hydrogen and oxygen to a burner, (ii) depositing the silicon dioxide to form a porous base material, and (iii) heating the porous base material in an electric furnace to form transparent quartz glass with a high degree of purity.
Manufacturing quartz glass using the VAD technique involves forming the porous base material by simultaneously rotating SiO2, generated by a flame hydrolysis reaction, and depositing the SiO2 on a starting material that can be lifted. During deposition, the position of the deposition tip of the porous base material is detected and the lifting speed is adjusted as the base material grows. As shown in Japanese Patent Application Publication No. 1-239033, however, when the lifting speed is held constant during deposition, it is necessary to detect the a deviation from the set value of the lifting speed at predetermined times and to correct the flow rate of the raw material gas such as SiCl4 according to the detected deviation, in order to obtain stable optical fiber base material with the desired refractive index distribution. In Japanese Patent Application Publication No. 3-242341, in order to maintain a constant lifting speed, the flow rate of the hydrogen supplied to clad burners adjacent to a core burner, from among a plurality of clad burners, is controlled.
Manufacturing quartz glass using the OVD technique involves forming the porous base material by rotating the starting material and moving the deposition burner back and forth along the starting material to deposit silicon dioxide around the starting material.
The porous base material formed in this way is then heated to about 1500 degrees Celsius in a heating furnace, resulting in transparent quartz glass. At this time, the inside of the furnace is often a helium atmosphere in order to decrease residual air bubbles in the glass. If necessary, a dehydration process is performed by heating the porous base material in an atmosphere containing chlorine at a temperature between 1000 and 1200 degrees Celsius, before being changed into the transparent glass.
When the VAD technique is used to manufacture a quartz glass optical fiber preform having a core with a high refractive index and cladding with a refractive index lower than that of the core, the core deposition burner is often supplied with germanium as the additive for increasing the refractive index of the quartz glass. The germanium is supplied as a compound, such as germanium tetrachloride (GeCl4). Germanium tetrachloride undergoes hydrolysis in the oxyhydrogen flame to create GeO2. SiCl4 and GeCl4 are liquid at a normal temperature, and are vaporized before being supplied using techniques such as bubbling with a carrier gas or heating to a temperature above the respective boiling points.
In manufacturing equipment using the VAD or OVD techniques, the flow rate of gas supplied to the burners is controlled by a thermal mass flow controller (MFC).
The hydrogen used here is made at a normal temperature or stored at a normal temperature. One idea involves heating and vaporizing liquid hydrogen stored in a low-temperature storage chamber to supply backup hydrogen when the supply of the above hydrogen is stopped.
In a VAD apparatus using hydrogen made at a normal temperature, when switching to hydrogen obtained by vaporizing liquid hydrogen due to a stoppage of the hydrogen supply, the lifting speed unintentionally increases by about 2%, which results in a thinner core diameter.
When the same hydrogen switching occurs in an OVD apparatus, the density of the porous base material decreases.
When the core diameter or the density of the base material changes in this way, it is impossible to achieve a stable optical fiber base material having the desired refractive index distribution, causing an increase in the chance of defects.